Lighting problem - indicators affect side light brightness

[Bruce Crowthorne]

My car has just failed it's MoT
When the indicators are on, the brightness of the sidelights varies in time with the flashing.
It is more noticeable on the rear since the side light and indicators are right next to each other (it's an S4). But when I check the voltages throughout the car the voltage varies everywhere! So the fronts vary in brightness as well, but you really don't notice it.

My initial assumption was a bad earth. I checked the earths in the boot and even tried adding new earths to each rear lamp. I went through the car and checked every earth - but all seem OK.

When I hook up a voltmeter the supply voltage varies in time with the flashing - by a bit less than half a volt.

I even went and checked for the same effect in a modern car (my Elise) but there is no observable variation in brightness.

Any ideas? I am prepared to clutch at straws ......

Thanks in advance
Bruce

PS One thought is to try another MoT place - yes, desperate!

[oldelanman]

The front side lamps and indicators share a common earth, my guess is that's where your problem is. The lamp earths are joined with bullet connectors in the nose and terminate in a ring terminal on the LHS radiator bracket - one or all of these connectors probably need cleaning up.
Good luck.

[theelanman]

I would guess some sort of short?......broken wire?.....
my modern car does this when a bulb goes.....the indicator rate changes......
in my youth I had a car that had been hit from behind....and when I braked lots of ineresting electrical stuff happened......it turned out that one of the bulb conectors was only just touching the body......

Im sorry I cant be much help but Im not good with Electrickery

[billwill]

No, I doubt if it is a short.

Clearly part of the shared circuit of the traficators and sidelights is of higher resistance than it ought to be.

This can either be the earthing lines or the supply lines.

First check the earth lines, modern looms seem to use wires that are rather thin; I personally would prefer to use thicker wire especially for Earth, and especially because we have fibre glass bodies and hence the earth wires are longer than those on metal cars.

If you haven't got a multi-meter, go buy one; digital meters are really inexpensive nowadays. With the lights powered OFF, use the OHMS section to measure the resistance between the cases of the lights and a firm contact on the chassis. You say you are seeing a voltage change of about 1 volt. A sidelight is 6 watts I think ? hence half an amp and a trafficator is 21 watts I think (hence 1.75 amps at 12 volts) so the combined current is around 2.25 amps. A 1 volt drop implies that your shared wiring has a resistance of 0.8 ohms.

Ohms below 1 ohm do tend to be a bit difficult to measure with a multi-meter, as its prods may have that order of resistance too, so maybe it might be better to measure the voltage drop instead with the lights on. To find out if the problem is the earthing measure the volts between the earth point on the lights and a good clean connection to the chassis. If it shows 1 volt there then your problem is in the earthing. If it is much smaller then the problem is in the common part of the supply.

Problems in the supply or earth lines can be caused by several things.
(a) wires too thin
(b) corroded connectors
(c) thin oxide layer on an apparently clean connector
(d) connector screws on 'hoop' connectors not done up tight
(e) dirty contacts in switches
(f) dirty contacts in fuse holders
(g) old tired fuses,

To find the problem you can start at the main supply and with the lights on and flashing, connect one side of the voltmeter to the main source (the brown wire at the starter solenoid) and then tap the other end of the voltmeter to every point along the supply line that you can reach, until eventually you reach the positive terminal of the lamp(s). When you find the 1 volt drop, you know that the problem is in that most recently checked portion of the circuit.

[billwill]

PS. On rereading I see that you have only a half a volt change when the trafficator comes on. So the excess resistance is only approx 0.4 ohms, rather small to measure directly.


I'm surprised that a mere half volt different makes a very discernibly brightness change in the sidelights. Maybe the bulbs are tired, try some new sidelight bulbs.

Do most of your voltage drop testing that I described above on the REAR lights, the wires to them are longer and more likely to have the problem.

[rcraven]

At a guess I'd agree with the suggestion that it's the wiring or connections in the front of the car. They are more likely to get damp and the bullets tend to corrode.

[Bruce Crowthorne]

Thanks for all the replies - really appreciated.

Let me reply to some of the specifics ...
1 Earths -I did new earths as I did my rebuild about 5 years ago. I mounted small spade connectors under the chassis bolts at both front and rear of the car.
However, I still tried running separate (duplicate) earths - but it made no difference.

2 Bulbs - checked and OK. However, they are old and have that grey / black deposit on the inside of the glass.
I like the idea of new bulbs so I will get some tomorrow morning

3 Feed or Return side? Now that I have (pretty much) eliminated the earth side I am assuming the issue is on the feed side.
The indicators and light don't share any wiring after the output of the voltage regulator, so I couldn't identify anything there.

4 Voltmeters - I am lucky to have several of varying ages. A digital one is no good for a varying voltage, so I am using an old "moving coil" meter which just about keeps up with the flasher timing.
I could really do with an oscilloscope - I keep lusting after one, but I would rarely use it ....

5 Voltage drops - The only place I can find a measurable voltage drop is across the main light switch, not much maybe 0.1V when on load.
But of course I don't have a spare! Mine are the black plastic rocker switch type - are they still available?

6 Many bits were renewed when I rebuilt the car - fusebox, most of the connectors etc. Basically anything to try and help avoid electrical problems ......

I am going to take it back to the MoT guy on Friday afternoon - I know it's not 100% fixed but I am not sure what to try next.
Fingers crossed ......

[billwill]

Sorry to say this, but if you are seeing a 0.5v variation at the sidelight bulb, but can only find a 0.1v drop variation anywhere else in the circuit,, that doesn't add up, you must have missed something.

Did you check across the fuse?

[lotusS2guy]

If you still cannot find the cause of the voltage drop, I suggest you replace the incandescent bulbs with the new LED bulbs. They use less current, so, all other things being equal, the voltage drop will decrease. The change in brightness should be undetectable.

Herb
26/4618

[Bruce Crowthorne]

Bill - yes I realise it doesn't add up - I admit I am not being rigorous enough. Desperation is setting in!

Herb - I like your thinking! But how will LED bulbs affect the flasher unit? They won't draw as much current so I am wondering if the flasher will actually "flash".....

[billwill]

Hopefully if you just replace the sidelights with LED bulbs, they will not be so drastically affected by the voltage variation and leaving the traficator bulbs as is, it should do enough current to operate the flasher.

[69S4]

I've had exactly the same problem a number of times over the years (although never failed the MOT on it - it must be really bad or you've got a tough tester)) and it's always been earthing issues. If it's been five years since you redid the electrics that's probably enough time for a bit of corrosion to creep in.

As Roger suggests I'd start at the front where the sidelights and indicators have their common earths and get the full blast of rain etc through the grill, but usually on my car it's not been one bad connector and everything else ok but rather a succession of slightly below par joints. A few minutes spent staring at the wiring diagram trying to imagine where the current could go if the normal earth wasn't available has helped me in the past - if nothing else it's a few minutes where you're not contorted into the nosecone trying to unscrew a connector with one hand while trying to hold the other end with a screwdriver outside the car. Now I know where osteopaths get their patients from.

[Bruce Crowthorne]

It passed - thank goodness.
Tested in bright sunlight - that probably helped.

Still isn't 100%, in fact I reckon the whole voltage supply varies enough to affect bulb brighness when the indicators are on. If I sit in my car with the courtesy light and the indicators on you can just about detect the variation in brightness of the courtesy light.
Does that happen with anyone elses?

As 69S4 mentioned, I think he is just a picky tester. Time to find a new one!

Thanks again for all the tips.
Bruce

[billwill]

I'd possibly be inclined to just tell the MOT tester,. Oh, dimming is normal in fibreglass cars nowadays, what with the price of copper wire.

[oldelanman]

Still isn't 100%, in fact I reckon the whole voltage supply varies enough to affect bulb brighness when the indicators are on. If I sit in my car with the courtesy light and the indicators on you can just about detect the variation in brightness of the courtesy light.
Does that happen with anyone elses?

Mine does that too but only noticable with the engine not running. I've found it's because the coil is drawing around 10 amps, pull the LT wire off the coil and the problem goes away.